1. Field of the Invention
The present invention relates to a process for the manufacture of thin ceramic cores for use in precision casting, and is particularly applicable to the production of turbine blades.
2. Summary of the Prior Art
The use of casting cores of so-called "ceramic" type is known in particular in applications in which it is necessary to obtain a set of strict quality characteristics and criteria, such as resistance to high temperatures, absence of reactivity, dimensional stability, and good mechanical characteristics. Among the applications which have such requirements are, in particular, aeronautical applications such as, for example, the casting of turbine blades for turbojet engines. The improvements in casting methods, developing from equiaxed casting to casting by directed or monocrystalline solidification, has also increased these requirements in connection with the cores, of which the use and complexity are determined by the search for high performance in the components to be obtained, such as in the case of hollow blades with internal cooling. The processes relevant to these fields of use are precision casting processes, and particularly the process known as lost-wax casting. In all cases, the core is used in the manufacture of hollow components. In the so-called lost-wax casting method, a core of ceramic material is held in position in the mold while metal is poured into the mold, the outer surface of the core forming the inner surface of an internal cavity of the finished product obtained in this way. The accuracy and dimensional stability of the core are therefore essential for conforming to the thicknesses intended for the cast metal components.
Examples of known compositions intended for the preparations of such cores are given in FR-A-2,371,257 and essentially comprise fused silica, powdered zircon, and cristobalite, which is a form of crystallized silica, together with a silicone resin as a binder. Additional elements, such as a lubricant and a catalyst, may be added in small quantities. The preparation method is also described.
In general terms, the cores used for casting the components and blades are composed of ceramic having a generally porous structure: these cores being produced from a mixture consisting of a refractory fraction (in the form of particles) and of a more or less complex organic fraction. Another example is described in EP-A-0,328,452.
In a manner known per se, the shaping of the casting cores, particularly when starting from thermoplastic pastes, can be carried out by molding, for example using press injection. This shaping is followed by a binder removal operation, during which the organic fraction of the core is eliminated by various known means, such as sublimation or thermal degradation, depending on the materials used. This results in a porous structure. A thermal core baking treatment is then applied to the refractory fraction to consolidate the porous structure. This treatment introduces a dimensional change in the form of a contraction, which is often nonisotropic, in the volume of the core, as compared with the initial shape.
At this stage, it may be necessary to reinforce the core so that it is not damaged in the subsequent cycle of use. For this purpose it is known to carry out an impregnation by means of an organic resin.
However, the improvements made to the cooling circuits of turbine blades have given rise to new demands entailing new requirements for the cores used in the manufacture of these blades by casting. In particular, it is difficult using conventional methods of manufacturing ceramic cores by press injection to obtain cores having a thickness of below 0.5 mm over a wide cross section.